Optical communications systems are becoming increasingly important in the high speed transmission of large amounts of information. A typical optical communications system comprises a source of modulated optical input signals, a length of optical fiber coupled to the source, and a receiver for optical signals coupled to the fiber. The input signals are typically in the form of digital pulses which are transmitted with minimum attenuation in guided modes along the axis of the fiber.
One difficulty with optical communications systems is dispersion. Different wavelength components of a pulse are transmitted with slightly different facility with the consequence that a sharp, symmetrical pulse at the input, after traveling many kilometers, becomes deformed and unsymmetrical. In the absence of preventative measures, a pulse will eventually degrade to a point where its initial location in a binary sequence is indeterminate.
It has been proposed that dispersion can be reduced by midspan spectral inversion of propagating pulses, i.e. at the midpoint of the fiber path inverting the pulse waveform so that the higher frequency portion has the shape of the lower frequency portion and vice versa (effectively a 180.degree. rotation of the pulse waveform about its center wavelength). As a result, after the inverted pulse travels over the second half of the communications path, the additional dispersion will reverse much of the distorting effect of the dispersion that occurred during the first half.
One approach to spectral inversion is through the use of a phenomenon known as four-wave mixing. When the pulse is co-propagated along a fiber with high power (5-50 mW) narrow band light near the pulse wavelength, a second pulse is produced at a wavelength slightly different from the original pulse. The frequency-shifted second pulse has an inverted waveform as compared to the initial pulse. Unfortunately, the four-wave mixing arrangements heretofore known require tens of kilometers of co-propagation and produce inverted pulses 10-25 dB down from the input pulse. Accordingly, there is a need for an improved four-wave mixer providing a stronger inverted pulse in a more compact arrangement.